Telephone system



P v. WELCH TELEPHONE SYSTEM Filed May 1, 1937 INVENTOR R WELCH Arrows/5KPatented Dec. 6, 1938 PATENT OFFICE TELEPHONE SYSTEM Paul V. Welch,Brooklyn, N.'Y.,'assignorto Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application May 1, 1937,Serial No. 140,104

5 Claims.

This invention relates to telephone systems and particularly tosupervisory circuits for use in connection with operatorscords or forother switching equipment.

.The objectsare to obtain better holding conditions for connectionsestablished by way of operatorscords or other connecting circuits; toinsure more positive operation for the supervisory devices intheseconnectingcircuits; to improve thetransmission to the station wherethe holding is controlled manually; and otherwise to improve theoperation and usefulness of systems of. the foregoing character.

The connecting circuits used in private branch exchanges are frequentlyprovided with direct current holding bridges connected across thetalking conductors to hold'the "supervisory equipment at the centraloffice while the private branch operator is obtaining a'wanted station.When the called private branch'exchange station answers, the-currentthat flows fromthe central ofiice over the loop of said called stationmust be suflicient to operate a supervisory "relay in the branchexchange operators cord circuit and to actuate the transmitter 'at thecalled station. But the holding bridge is in parallel with the calledstation and acts as a shunt, preventing full current from :flowingthrough the station transmitter and the supervisory relay, which areconnected in :series with the station. If the bridge is made oflowresistance, insuring a good holding condition, the shunting eitectmay:prevent the operation of the supervisory relay in the private branchcord circuit when the called station answers. In case the holding bridgeis applied manually by the operator, the bridge may shun-t toomueh ofthe current away from the transmitter in the event the operator forgetsto remove thebridge before the parties begin conversation. On the otherhand, if the resistance of the bridge is madehigh to insure a safeoperating margin for the supervisory relay and to insure sufiicientcurrent for conversation should the bridge be left across the line, itsresistance may prevent the flow of enough current to hold thesupervisory equipment at the central oflice.

According to the present invention these difiiculties are overcomeand-adequate operating-currents are obtained for the holding andsupervisory and transmitter requirements by means of a novel circuitarrangement in which a variable resistance is included in circuit withthe inductive direct current bridge circuit, and which functions to givethe bridge a low resistance for hold-- ingpurpos'es Iand-a highresistance when current is needed forsupe'rvisory or transmitterpurposes.

More specifically this feature is realized by connecting a resistanceelement having a high negative temperature coeiiicient of resistance inseries with the windings of the inductive holding bridge acrossthe'private branch exchange cord. Normally, =th'e'resistanceof theelement 'is high, but, when the branch exchangeoperator answers a callover a trunk from central ofiice, current flowing over the trunk fromcentral office and through the bridge inher cord heats the'elem'ent andlowers its resistance. This lowering of the bridge resistance allowsfull current to flow in the-connection andinsures the holding of theequipment at the central ofiice so as notto give the operator there'at afalse disconnect signal While the branch exchange operator is setting up'the desired connection. When thecalled station in the branch exchangeanswers, the loop of this station is closed in parallel with said bridgeand diverts some of the current therefrom. 'Ihis l-oss of current causesthe resistance element to increase its resistance, causing more and morecurrent to flow through the station loop. This increased flow of currentthrough the loop insures adequate current to the station transmitter andthe operation'of the supervisory relay in the branch exchange cord,advising the operator that the called station has answered, andthecalled station loop now acts in place of the bridge to hold theconnection at the central oflice. When the called party hangs up his'receiver, the opening of his loop removes the shunt from the holdingbridge, the resistance element therein again assumes a low resistance,and suf-- ficient current is maintained in the trunk to hold theconnection at central ofiice until the branch operator breaks theconnection by withdrawing her cord circuit.

The foregoing and other features of the invention will 'be discussedmore fully in the following detailed specification.

In the drawing accompanying the specification:

Fig. '1 illustrates a central office and a private branch exchange witha trunk interconnecting them and shows operators" cord circuits at bothends of the trunk, the branch exchange cord circuit being equipped witha variable-resistance holding bridge; and

Fig. 2 shows the variable-resistance holding bridge applied to-the trunkcircuit in a private branch exchange equipped with key connectingcircuits instead of operators cord circuits.-

The resistor connected in the bridge circuit may be constructed in anysuitable manner, and any substance having the necessary resistancecoeificient may be used. Materials, such as silver sulphide and boron,having large negative temperature coefficients of resistance areparticularly Well suited for this purpose. Resistors made of thesematerials have high resistance at normal temperatures and rapidly reducetheir resistance by large amounts when they are heated by currentflowing through them. The construction and characteristics of resistanceelements of this type are more fully discussed in the British Patent472,144, accepted Sept. 17, 1937. Also other types of resistors, such asthose made of copper oxide, which vary their resistance with relation tocurrent may be used instead of those mentioned above.

While the invention has been illustrated as applied to the holding andsupervision of private branch exchange connections, it will be celar to7 those skilled in the art that it is similarly applicable to any typeof circuit connection where it is desired to have the connectionoriginally offer a low resistance and later to increase its resistanceautomatically in response to the closure of a parallel circuit. As anexample of another use for the invention it might be applied to aselector or connector switch in an automatic telephone system.

Referring now to the drawing and particularly to Fig. 1, the operator atthe central office is provided with a cord circuit C by means of whichshe establishes connections, including those over the trunk T to theprivate branch exchange.

The cord C has the usual equipment, much of which is omitted in thedrawing, including a repeating coil I4 and supervisory relay I. At thebranch exchange the operators cord CI likewise includes the listening,ringing, supervisory, and other equipment usually provided for cords ofthis type. The holding bridge, hereinbefore discussed, for the cord C--Icomprises a choke or inductance coil having a magnetic core 2 and twoinductive windings 3 and 4. A variable resistance element 5 is connectedbetween the two windings 3 and 4 and forms a part of the conducting pathfor the flow of current through the bridge circuit. The cord circuit C-Iis also equipped with a supervisory relay 6 and lamp I9 to give theoperator supervision over lines, such as the station line S, to whichshe makes connections by means of the cord. It will be noted that thebridge circuit in the cord circuit CI forms a closed path for the flowof current from the source in the central oflice cord circuit C over thetrunk T to the private branch exchange cord. It will also be noted thata station line S, when the cord circuit CI is connected thereto, is inparallel or shunt relation to the bridge in the cord circuit CI. Withthe receiver off the hook at the station S, this parallel circuit isclosed, and current from the central office battery flows over thetalking conductors and through the loop of said station line.

A description of the operation will now be given. Assume that thesubscriber of line A terminating in the central oifice wishes to obtainconnection with the station line S in the distant private branchexchange. The operator at the central office, having answered thecalling line A, inserts the plug 1 of her cord circuit C in the jack 8of a trunk T and causes a calling signal (not shown) to appear at thejack 9 in which the trunk terminates at the branch exchange.

The insertion of plug 1 closes a circuit from battery through thesupervisory lamp l6, resistance I'I, sleeve contacts of the plug I andjack 8 and thence to ground. The lamp I6 burns in this circuit.

The branch exchange operator replies by inserting the plug II] of cordCI in the jack 9. The insertion of plug I closes a current supplycircuit from battery in the central office through the lower rightwinding of the repeating coil I4, winding of the supervisory relay I,ring contacts of plug I and jack 8, over the lower conductor of thetrunk T, through the ring contacts of jack 9 and plug I0, throughcontacts of the listening key II, thence through the holding bridge,including coil 4, resistance element 5, and coil 3, thence throughcontacts of the listening key II, tip contacts of plug I0 and jack 9,over the upper talking conductor of the trunk T, tip contacts of jack 8and plug I through the upper right winding of repeating coil I4 toground. When this circuit is first closed, the holding bridge offers ahigh resistance to the flow of current because the element is at itsnormal temperature and consequently at its high-resistance value. Theflow of current through the bridge, however, and through the element 5quickly raises its temperature, and its resistance then falls rapidly toa low value. This permits full current to flow immediately over thecircuit traced, which includes the winding of the supervisory relay I.Relay I receiving full current operates its armature and short-circuitsthe supervisory lamp II; to extinguish it. The private exchangeoperator, by means of her listening key I I, connects her telephone setI2 across the cord circuit and challenges the central office operator.The opera tion of listening key I I opens the cord circuit, deprivingthe holding bridge of current, and the resistance element 5 quicklyassumes its highresistance value. The supervisory relay I in the centraloifice, however, is now held by the con nection through the operatorstelephone set in the branch exchange. Having obtained the desiredinformation from the central office operator, the branch operatorrestores the listening key II, reclosing the holding circuit previouslytraced through the holding bridge, and inserts the calling plug I3 inthe jack I8 of the wanted station line S. The insertion of the plug I3in jack I8 closes a circuit from battery through the supervisory lampI9, resistance 20, sleeve contacts of plug I3 and jack I8 to ground. Thelamp I9 is illuminated in this circuit. In any suitable manner thebranch operator signals the station line S by applying ringing currentthereto.

When the party at station S replies by removing his receiver from theswitchhook, a direct current path, including the supervisory relay 6, isclosed in parallel or shunt relation to the holding bridge in the cordCI. As previously explained the full current flowing over the trunk fromthe central oflice has, up to this point in the operation, passedthrough the holding bridge in the cord C--I. This current now divides,and some of it flows over the parallel path through the loop of thestation line S. The resulting diminution of current in the holdingbridge causes a corresponding increase in the resistance of the element5. This permits still more of the current to pass through the substationloop with a further reduction in the current flowing through the bridgecircuit. In this way the variable resistance element 5 automaticallyincreases its resistance as a result of the closure of the shunt circuitover the station line, and substantially all of the current suppliedfrom the central oflice is directed over the station line and throughthe supervisory relay 6. Consequently the relay 6 obtains a fulloperating current, and the station line S is supplied with adequatecurrent for transmission, although no physical opening has been made inthe bridge circuit across the cord C-I. Relay 6 closes a short circuitaround the lamp I9 to extinguish it as a signal that the called stationS has answered.

When the party at the called station S hangs up his receiver at the endof conversation, the loop circuit in parallel with the holding bridge isopened and the supervisory relay 6 releases to cause the illumination ofthe lamp I9 as a disconnect signal. The opening of the shunt through thestation loop places full potential across the bridge circuit, and alarger value of current again flows through said bridge and the variableresistance element 5. The element 5 reduces its resistance, and fullcurrent immediately flows through the bridge to hold the supervisoryrelay I in the cord C in its operated condition. When finally the branchexchange operator disconnects by removing the plug lll from the jack9,-the relay l releases, and the lamp I6 lights as a disconnect signalto the central office operator.

Referring now to Fig. 2, the trunk circuit 22 incoming from the centraloffice terminates in the contacts of keys at the private branchexchange. The attendant completes an incoming call by first operatingthe key contacts 23 and 24 to connect the calling trunk 22 to one of herconnecting circuits 25. She then manipulates the key contacts 26 and 21to complete the connection to the called line 28 after having receivedthe necessary information from the calling party.

Should the private branch exchange operator wish to hold a connection onthe trunk 22, she may do so by closing her holding key 29 to complete aholding bridge across the tip and ring conductors of the trunk. Thisholding bridge includes an impedance element 30 and a variable resistor3|. The holding bridge including the element 3| serves the same purposeas the bridge described .in connection with Fig. 1. When the bridgealone is across the line, the resistance of element 3| is low, andsufficient current flows to hold the central office connection. When,however, the called station answers, the resistance of element 3|increases, and the bridge does not act as a substantial shunt on thetransmitter at the called station should the attendant operator forgetto remove it during the conversational period.

What is claimed is:

1. In combination, a telephone line, a switching circuit for makingconnection with said line, a bridge connected across the conductors ofsaid switching circuit, means for closing said telephone line toestablish a path including said line in parallel to said bridge, asource for supplying current over said switching circuit to said bridgeand to said line, and a variable-resistance element responsive to theflow of current in said bridge to lower the resistance of the bridge andresponsive to the flow of current in said line to raise the resistanceof said bridge.

2. In combination, a telephone line, a connecting circuit for makingconnection with said line, a bridge across the conductors of saidcircuit, means for closing a path including said line in shunt relationto said bridge, a source for supplying current to said bridge and tosaid line, and a variable-resistance element responsive to current insaid bridge while said shunt path is open to lower the resistance ofsaid bridge and responsive to the reduced flow of current through thebridge caused by the closure of said shunt path to raise the resistanceof said bridge.

3. The combination with a telephone line of a cord circuit for makingconnection therewith, a bridge connected across the conductors of saidcord, means for closing a path including said line in parallel relationwith said bridge, a source for supplying current to said bridge and tosaid line, and a variable-resistance element in said bridge responsiveto the flow of current to lower the resistance of said bridge andresponsive to the reduced flow of current through the bridge when saidparallel path is closed to raise the resistance of said bridge.

4. The combination with a telephone line of a cord circuit for extendingconnections to said line, a bridge connection across the conductors ofsaid cord, a supervisory relay, means for closing a path including saidline and said supervisory relay in shunt relation to said bridge, asource for supplying current to said cord circuit, and avariable-resistance element serving to reduce the resistance of saidbridge when said shunt path is open and to increase the resistance ofsaid bridge when said shunt path is closed.

5. The combination in a telephone system of an incoming line having asupervisory relay associated therewith, an outgoing line, a cord circuitfor interconnecting said lines, a bridge connected across the conductorsof said cord circuit, means for closing a path including said outgoingline in shunt relation to said bridge, a current source and circuitmeans for supplying current through the winding of said supervisoryrelay to said cord circuit, and variable-resistance means effective whensaid shunt path is open to lower the resistance of said bridge andpermit the full flow of current from said source through said bridge andeffective when said shunt path'is closed to raise the resistance of thebridge to cause the current from said source to flow over said shuntpath.

PAUL V. WELCI-I.

